1. Field of the Invention
The present invention relates to a method for inspecting pattern shapes, and improvements in a device therefor.
2. Description of the Prior Art
The manufacturing process of a semiconductor device includes a process in which necessary patterns are transferred on a substrate with a mask pattern (reticle) comprising a shielding portion and a transparent portion. Accuracy of the transferred pattern image by the mask pattern significantly influences the performance of a semiconductor device, and thus it is required that a highly accurate pattern image is transferred by the mask pattern.
Conventionally, the method for inspecting the transferred pattern image by the mask pattern is disclosed in Japanese Patent Publication No. B63-56702 specification, etc. In this conventional example, existence of abnormality in a pattern to be inspected is detected by comparing an inspection image which has been picked up by irradiating a laser beam to the pattern to be inspected with designed data.
However, this conventional example has disadvantages in that changes of a laser light source due to the lapse of years or variations of scanning means to scan the surface of a pattern to be inspected with a laser beam may give rise to variation in optical intensity of the inspection light, and thus the optical intensity profile of scan transmitting light within a scanning range may change, or holistic intensity drops may cause the entire gradation of an inspection image to become unusable.
Consequently, the inspection accuracy of the mask pattern may decrease.
In addition, when using the diffraction effect of an acousto-optic element as scanning means, the diffraction effect varies in accordance with the temperature of the acousto-optic element, and consequently, the optical intensity per process area may vary. This variation in optical intensity due to the temperature of the scanning means is generated mainly in a sub-scanning direction. In addition, also as for a primary scanning direction, the optical intensity may vary within a primary scanning time period (e.g., 6 xcexcsec. according to changes in the configuration of the optical system,.
Object of the Invention
An object of the present invention is to reduce the disadvantages which such conventional examples have, and especially, even though there are variations in the optical intensity of a laser beam, to remove influence from this variation components and thus maintain inspection accuracy.
Moreover, another object of the present invention is to remove the influence of both variation in the optical intensity in the primary scanning direction of a laser beam and variation in the optical intensity in the sub-scanning direction thereof, maintain inspection accuracy even though there are changes due to the lapse of years or changes in temperature, and thereby improve the yield factor of semiconductor products.
In addition, another object of the present invention is to maintain this inspection accuracy at low costs, as well as at a high speed and in a stable manner.
The present invention comprises a light source of irradiating inspection light, a scanning unit for scanning the inspection light, an inspection light dividing unit for dividing the inspection light scanned by the scanning unit, a monitor light detecting unit for receiving a part of the inspection light divided by the inspection light dividing unit and converting it to a monitor signal, a transmitting light detecting unit for receiving a transmitting light, which has transmitted through a pattern shape, and converting the transmitting light into a transmission detecting signal, a waveform shaping unit for removing an alternate current component, within the primary scanning time period of the aforementioned scanning unit, of monitor signals outputted from the monitor light detecting unit at each round of the primary scanning to convert the monitor signals into a rectangular-shaped wave of an approximately constant value.
Moreover, the present invention further comprises a first correction unit for dividing the transmission detecting signal by the rectangular-shaped wave, which has undergone shaping in the waveform shaping unit, as a divisor group, and a second correction unit for dividing the transparent detecting signals corrected in the first correction unit with predetermined reference transmitting signals as a divisor group and outputting the results of said division as an inspection image.
Here, the inspection light is divided, and one part is treated as the monitor signal and the other part is treated as the inspection light. And the inspection light transmits the pattern shape such as reticle or mask pattern, etc. The pattered portion shields lights, and thus the transmitting light will represent the shape of the pattern. In general, among pattern shapes the portion not patterned is a glass surface, and thus the inspection light transmits through the glass surface to become a transmitting light.
Depending on the primary scanning direction, intensity of the transmitting light undergoes slight changes. This is caused by changes in intensity of the inspection light due to scanning angle of the scanning unit, and changes in intensity of the transmitting light as a result of transmitting through an optical system. For the purpose of correcting the changes in intensity which appear in this primary scanning direction every time, a pattern shape configured with only glass surface without comprising any pattern in advance and the like should be equipped and the transmitting reference signal should be measured. The second correction unit divides the transmission detecting signal by the transmitting reference signal, and thus corrects changes in intensity appearing in the primary scanning direction every time.
In addition, the optical intensity of the inspection light varies in the sub-scanning direction as well. For example, where the scanning unit and the inspection light dividing unit are temperature-dependent, intensity of the inspection light varies in the sub-scanning direction in accordance with changes in temperature. In addition, it is expected that the output of the light source will be changed after the transmitting reference signal has been measured. For purpose of removing such variation of optical intensity of the transmitting light appearing during a period longer than the time range of primary scanning, the first correction unit divides the transmission detecting signal with the monitor signal. Then, for the variation of optical intensity in the sub-scanning direction, if any, the transmission detecting signal and the monitor signal vary at the same ratio, and therefore, dividing-calculation results will remain constant.
And at this time, the correction purpose in the first correction unit occurs during a period longer than the primary scanning period, the waveform shaping portion removes the alternate current components during the period of one round of the primary scanning from the monitor signal, and leaves only the direct current components. In particular, for example, the peak within a short time period xcex4t-immediately after the monitor signal rises is maintained, and this peak value is outputted during the period of the said one round of primary scanning. In addition, using this approximately rectangular-shaped monitor signal, the first correction unit implements dividing-calculation, and thus only the level of transmitting signal appearing in the sub-scanning direction can be corrected. In addition, the monitor signal is shaped into a rectangular-shaped wave so that it is not subject to the influence of the optical intensity which takes place only in the monitor signal during the primary scanning period immediately after the inspection light has been divided. Especially, in spite that the fracture takes place at the time of trailing of the monitor signal, the level of the transmission detecting signal can be corrected without being influenced by this fracture. On the other hand, using the transmitting reference signal having transmitted through the glass surface, the second correction unit is corrected, and thus the variation components of the transmission detecting signal appearing in the primary scanning direction can be removed under the state including the variation of the optical intensity due to transmitting through the glass surface.